My watch list  

Contact process

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Vanadium(V) oxide is the catalyst employed.

Additional recommended knowledge

This process was patented in 1831 by the British vinegar merchant Peregrine Phillips, as a far more economical process for producing sulfur trioxide and concentrated sulfuric acid than the previous lead chamber process method used for producing sulfuric acid.


The process can be divided into three stages:

  1. Preparation and purification of sulfur dioxide
  2. Catalytic oxidation (using vanadium oxide catalyst) of sulfur dioxide to sulfur trioxide
  3. Conversion of sulfur trioxide to sulfuric acid

Purification of air and SO2 is necessary to avoid catalyst poisoning (ie. removing catalytic activities). The gas is then washed with water and dried by H2SO4.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulfur dioxide and oxygen then react in the manner as follows:

2 SO2(g) + O2(g) ⇌ 2 SO3(g) : ΔH = −197 kJ mol−1

To increase the reaction rate, high temperatures (450 °C), high pressures (10)atm), and vanadium(V) oxide (V2O5) are used to ensure a 99.5% conversion. Platinum would be a more suitable catalyst, but it is very costly and easily poisoned. The catalyst only serves to increase the rate of reaction - it has no effect on how much SO3 is produced.

Hot sulfur trioxide passes through the heat exchanger and is dissolved in concentrated H2SO4 in the absorption tower to form oleum:

H2SO4(l) + SO3 → H2S2O7(l)

Note that directly dissolving SO3 in water is impractical due to the highly exothermic nature of the reaction. Acidic vapour or mists are formed instead of a liquid.

Oleum is reacted with water to form concentrated H2SO4.

The average percentage yield of this reaction is around 30%.

H2S2O7(l) + H2O(l) → 2 H2SO4(l)


  • Jim Clark (2002). The Contact Process. Chemguide.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Contact_process". A list of authors is available in Wikipedia.
Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE